This RO1 application requests funds to develop and validate a novel outcome measure targeting the behavioral inflexibility in children with intellectual and developmental disabilities (IDD). This behavioral domain encompasses a host of closely related clinical sequelae that can be profoundly disabling and are common in IDD such as (a) an overall inflexible temperament or behavioral style that underlies discrete idiosyncratic forms of repetitive and rigid behavior manifestations (e.g., inability to tolerate changes in the environment, stereotyped uses of objects); (b) associated mood and behavior problems that arise when this inflexible temperament and its manifestations cannot be accommodated; and (c) interference with learning and adaptation that stems from rigidity / inflexibility as even treatmen itself can represent intrusion and change for an inflexible child (e.g., a new pill, a new therapy exercise). In recent years there have been large-scale drug trials and small behavioral trials targeting discrete behaviors that fall under the domain of repetitive inflexible behavior. However, the measures included in many of these clinical trials have little to no evidence of their ability o detect change over time. In fact, while several instruments exist that measure the discrete behavioral endpoints of this overall construct of rigidity / inflexibility, no instrument exists tht adequately extends much beyond a behavior checklist approach. The overall objective of this application is to develop and validate a new measure, the Repetitive and Inflexible Behavior Inventory (RIBI), with the primary goal of developing a measure that is sensitive to change over time. The RIBI is designed as a parent report measure for children (ages 3 -18) with IDD. The rationale is that a novel measure is needed to (a) capture the range of repetitive inflexible behaviors that are found in children with IDD across a variety of idiopathic and genetic disorders, (b) measure the functional impact of these behaviors on the family's daily life, and (c) detect change over time when these behaviors are the target of intervention. Thus, the specific aims of this project are to (1) examine the psychometric properties of the RIBI for children and adolescents with autism spectrum disorder (ASD), establishing reliability, factor structure, content and construct validity; (2) evaluate the RIBI's ability to measure change over time in children with ASD; and (3) examine the psychometric stability of the RIBI when applied to children and adolescents with Fragile-X and Prader-Willi syndromes. To accomplish these aims, a multisite study will be conducted using recommended instrument development methodologies, such as focus groups, expert panels, and online surveys (n=1500). This team is in an ideal position to develop the RIBI because of our prior experience with measure development, and the conduct of clinical trials targeting repetitive inflexible behavior. The expected deliverable i an outcome measure that is appropriate for IDD populations and minimally burdensome to complete in order to be useful for multisite clinical trials.